International patent application WO 93/11892 discloses a method and a device for non-gravity casting of moulds. In particular, the patent relates to green-sand moulds in a mould string in a mould-string plant with light metal, in which the metal is pumped by means of an electro-magnetic pump from a holding furnace via a heated ceramic tube through a casting nozzle. The casting nozzle is in sealing abutment with a bottom inlet in the mould and is connected to the mould cavity. After casting the mould, the inlet is closed by means of a displaceable shutter core and the electro-magnetic pump is reversed so that the metal is sucked a distance backwards in the casting nozzle. The casting nozzle may then be removed from the mould and brought into abutment against the inlet of the next mould in the mould string.
Applicants' co-pending international patent application No. PCT DK95/00204 deals with a number of methods of closing the inlet of a mould after casting of moulds in the manner described above.
When the metal is sucked back into the casting nozzle, air is sucked along with it e.g. through the pores in the moulding sand around the inlet of the mould against which the nozzle is in sealing abutment. This takes place rather slowly and thus prolongs the casting process. Also, atmospheric air can bubble up through the metal in the casting nozzle and cause oxidation of the metal, as well as oxidation of the metal surface in the nozzle. Metal can also adhere to the internal surface of the passage in the nozzle. On casting the next mould, metal oxides created in this way may be led into the mould cavity of the mould and be included in the casting thereby reducing the quality of the casting possibly to an extent resulting in its rejection. Moreover, oxides may settle around the opening of the nozzle and prevent the sealing abutment against the next mould of the mould string.